The present invention relates to a non-linear optical film having a non-linear optical characteristic for use in a device for changeover of a signal optical path or for turning-ON/OFF of a signal.
The conventional switching of light information has required a photo-electric conversion means for once converting light information into electrical information and an electro-photo conversion means for reversely converting the electrical information into light information at each relay point. In order to build up a higher-speed light communication system, optical switches (mechanical type optical switches, planar light waveguide type optical switches, mirror type optical switches, bubble type switches, etc.) for directly switching the light information have been developed as described in xe2x80x9cNikkei Electronicsxe2x80x9d No. 8, Jan. 29, 2000 (non-patent document 1).
For the purpose of developing an optical switch capable of much higher-speed switching, there has been an attempt to achieve a further higher-speed response performance by use of a material having a non-linear optical characteristic. It has been reported that a high-speed and high non-linear optical characteristic can be attained by use of a vitreous thin film in which semiconductor particulates of a ferromagnetic oxide are dispersed, by Japanese Patent Laid-open Nos. 5-224262 (patent document 1) and 2002-72264 (patent document 2).
The optical switch using the thin film as above-mentioned has posed a problem that at the time of changeover of a signal optical path, absorption and/or scattering due to the thin film occurs, causing the attenuation of the signal.
Accordingly, it is an object of the present invention to provide a thin film that solves the above-mentioned problem, and an optical device using the same.
According to the present invention that solves the above-mentioned problem, there is provided a thin film comprising oxide particles having a corundum type crystal structure and amorphous grain boundaries in the surroundings of the oxide particles. The corundum type structure means a structure in which one more kind of atom is contained in a vacancy in a hexagonal crystal, which can be distinguished by a diffraction image obtained by a TEM (transmission electron microscope) or wide angle X-rays. Such a thin film has a refractive index variation sufficient for application to optical devices, and shows little loss of signals.
When having a mean particle diameter of not more than 25 nm, the oxide particles provide good switching characteristics. The oxide particles, which contain aluminum oxide, iron oxide or gallium oxide, are suitable since they further reduce the loss. In addition, iron oxide is suitable since it increases a variation in refractive index of the thin film. The material oxide constituting the oxide particles can be added in a weight ratio of from 30%, inclusive, to 95%, exclusive, based on the total weight of the thin film.
Rare earth element oxides can be added to the metallic oxide material in a weight ratio of from 1 to 20%. The addition of a rare earth element in the present invention is preferable since it promises a property for a high-speed change of refractive index.